The light transmitting performance of an optical fiber is highly dependent upon the properties of the polymer coating that is applied to the fiber during manufacturing. Typically a dual-layer coating system is used where a soft (low modulus) primary coating is in contact with the glass fiber and a hard (high modulus) secondary coating surrounds the primary coating. The secondary coating allows the fiber to be handled and further processed, while the primary coating plays a key role in dissipating external forces and preventing them from being transferred to the fiber where they can cause microbend induced light attenuation.
The functional requirements of the primary coating place several constraints on the materials that are used for these coatings. In order to prevent bending and other external mechanical disturbances from inducing losses in the intensity of the optical signal transmitted through the fiber, the Young's modulus of the primary coating must be as low as possible (generally less than 1 MPa, and ideally less than 0.5 MPa). To ensure that the modulus remains low when the fiber is exposed to low temperatures during deployment in cold climates, the glass transition temperature of the primary coating must be low (generally less than 0° C., and ideally less than −20° C.) so that the primary coating does not transform to a rigid glassy state. Also, the tensile strength of the primary coating, must be high enough to prevent tearing defects when drawing the fiber or during post-draw processing of the coated fiber (e.g. when applying ink layers or bundling the fiber to form cables). Obtaining the necessary tensile strength is challenging because tensile strength generally decreases as the modulus decreases. This means that the objectives of achieving low modulus conflicts with the objective of achieving high tear strength. Lastly, to ensure uniformity in the thickness of the primary coating, the composition from which the primary coating is formed is applied to the fiber in liquid form. A liquid primary coating composition flows to provide uniform coverage of the fiber to promote uniformity of thickness in the cured state. It is similarly beneficial to employ a liquid phase secondary coating composition. It is desirable, however, to apply a liquid secondary coating composition to the cured state of the primary coating to prevent mixing of liquid phases and potential contamination of the primary coating with components of the secondary coating (and vice versa). To achieve this goal while maintaining high draw speeds, the liquid phase primary coating composition must be capable of curing quickly to form a solidified primary coating having sufficient integrity to support application of a liquid secondary coating composition.
To meet these requirements, optical fiber coatings have traditionally been formulated as mixtures of radiation curable urethane/acrylate oligomers and radiation curable acrylate functional diluents. Upon exposure to light and in the presence of a photoinitiator, the acrylate groups rapidly polymerize to form a crosslinked polymer network which is further strengthened by the hydrogen bonding interactions between urethane groups along the oligomer backbone. By varying the urethane/acrylate oligomer, it is possible to form coatings having very low modulus values while still providing sufficient tensile strength to minimize damage during the draw or post-draw processing. Numerous optical fiber coating formulations have been disclosed in which the composition of the radiation curable urethane/acrylate oligomer has been varied to achieve different property targets.
An important attribute of optical fibers is consistency of performance. The deployment life of an optical fiber typically extends for years, or even decades, and it is essential that the performance of the fiber remain true and not degrade over time. A key attribute of the primary coating is its ability to mitigate signal attenuation resulting from bending or other stresses applied to the fiber. To maintain minimal stress-induced signal attenuation, it is necessary for the modulus of the primary coating to remain constant over time. In order to achieve a desired modulus for the primary coating during initial fabrication, curing conditions are often controlled to control the degree of cure of the primary coating composition. As the degree of cure increases, the primary coating becomes more rigid and achieves a higher modulus. To maintain the modulus within a desired range, the curing reaction of the primary coating is often arrested before completion and the degree of cure is less than 100%. As a result, subsequent processing events that act to increase the degree of cure of the primary coating lead to unintended increases in the modulus of the primary coating and deterioration of the ability of the primary coating to provide the buffering of external forces needed to prevent stress-induced signal losses in the fiber.
Since many primary coating compositions are cured by UV light, exposure of the primary coating to UV light during subsequent processing steps presents the risk of increasing the modulus of the primary coating due to UV-induced reactions that increase the degree of cure. Potential exposure of the primary coating to UV light during processing occurs when applying subsequent layers to the fiber. A secondary coating composition is often applied to the cured primary coating. To form a secondary coating from the secondary coating composition, it is necessary to cure the secondary coating composition with UV light. The UV light can pass through the secondary coating composition to reach the primary coating and alter the degree of curing of the primary coating. Similarly, application of ink layers or buffering layers may also subject the primary coating to exposure from UV light. Exposure of the primary coating to additional intensity of UV light leads to overcuring of the primary coating as well as unpredictability and inconsistency in the modulus and performance of the primary coating. It is accordingly desirable to develop primary coatings with a modulus that is stable to subsequent exposure of the primary coating to UV light during fiber processing.